1. Field of the Invention
The present invention relates to a noise canceling apparatus for weight measurement, and to a noise canceling method for weight measurement using a digital filter.
2. Description of the Background Art
A measured signal, outputted from a load detector such as a strain gauge load cell or a force balance, contains various types of noises resulting from a measuring system. In the case of load measurement in a carrier system, noise resulting from rotation vibration by a motor for driving a belt conveyer or by a carrier roller, or electrical noise resulting from a commercial power source, may be caused. These noises are generally superposed on the measured signal as vibration noises of a fixed period. A measuring system including a structure has a frequency specific to this structure. When external vibration falling within the range containing this specific frequency acts on the measuring system, the external vibration is amplified at about the specific frequency, and thereafter, emerges into the measured signal as a vibration noise. The vibration noise of a fixed period and the vibration noise resulting from the specific frequency will be collectively referred to as a “fixed-period vibration noise”.
For cancellation of the fixed-period vibration noise, digital filters have been used in recent years that are unlikely to be subject to secular changes and ambient surroundings, bearing relatively easy-to-change filter characteristics. Among those digital filters, a finite impulse response (FIR) digital filter, exhibiting excellence especially in stability, has been preferred.
For cancellation of the fixed-period vibration noise in the measured signal, a low-pass filter has conventionally been used that provides a sufficient attenuation in a frequency band corresponding to the fixed-period vibration noises. Using such low-pass filter, the fixed-period vibration noise can be attenuated in a relatively short period. The exemplary technique of which is introduced in Japanese Patent Publication No. 63-052684 (1988), titled “Method of Canceling Fixed-Period Vibration Wave”. In this method, a moving average filter is used which provides the number of times of sampling as the number of its taps, for a period corresponding approximately to an integer multiple of the period of the fixed-period vibration noise. However, this number of taps may vary according to a period (frequency) of a noise, possibly causing delay time in the moving average filter. Further disadvantageously, when the number of times of sampling, for a period corresponding to an integer multiple of the period of the fixed-period vibration noise, does not coincide with the number of taps, error resulting from such discordance may be likely.
In order to handle various fixed-period vibration noises having their respective frequencies including power supply noise of a relatively high frequency and mechanical vibration noise of a relatively low frequency, for example, the method disclosed in the foregoing publication is required to prepare a multistage moving average filter to be responsive to each type of noise. When various noise vibrations are to be attenuated by a single-stage filter, this filter is required to contain a plurality of stop bands (notches) to be respectively responsive to these noise vibrations.
In a carrier and measuring system, an object to be measured may be carried at a variable speed, causing a motor or a roller constituting the system to vibrate at a frequency that suitably varies according to the speed. In this case, a multistage filter should be prepared to respond to a frequency variation of a fixed-period vibration noise. Further, various types of filter characteristics have been required to respectively respond to the noise having a varying frequency such as the one resulting from a motor or a roller, and the noise having a virtually constant frequency such as the one resulting from a power supply frequency.